JP4369599B2 - Optical fiber body and optical module including the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is suitably used for an optical communication device, an optical measurement sensor, and the like, and includes an optical fiber body that optically couples (optically connects) an optical fiber with another optical isolator, an optical element such as a wavelength filter, and the like. The present invention relates to an optical module.
[0002]
[Prior art and its problems]
With the development of optical technology, optical signal and optical energy transmission means using optical fibers are actively used in fields such as optical communication and optical measurement. In such a system, it is necessary to optically couple a light source, a light receiver, a filter, an optical element for sensing, and an optical fiber. When a wavelength filter or an optical element for sensing is inserted into the optical fiber transmission line, the coupling loss must be minimized.
[0003]
As shown in FIG. 5, the optical system is most often used so far by aligning the optical element 4 such as the single mode fiber 1 for transmission, the lens 8, the optical element 4 such as the wavelength filter, the lens 8, and the single mode fiber 1 in this order. It has been. Reference numeral 9 denotes a holder for holding the lens, and 10 denotes a package.
[0004]
In the above optical system, the optical element 4, the lens 8 and the like are independent components and are aligned after being separately fixed to the holder. Therefore, there is a problem that the number of components is large, adjustment is complicated, and the size is increased. there were.
[0005]
In order to cope with this problem, as shown in FIG. 6, two core expansion fibers 11 are used without using a lens, and the optical element 4 is sandwiched between them using, for example, an arrangement (for example, JP, 9-54283, A).
[0006]
Such a core expansion fiber has a large tolerance of defocus (corresponding to the distance between the core expansion fibers in the direction parallel to the optical axis), so even if an optical element is installed between the optical fibers, the coupling loss is increased. Less is. It is important to adjust the misalignment of the core expansion fiber (deviation in the direction perpendicular to the optical axis). However, since the outer shape is the same as that of a normal optical fiber, it can be inserted into a ferrule and connected to a normal optical fiber. However, since no step or the like occurs in the connecting portion, the axis alignment of the core expansion fiber can be ensured with extremely high accuracy by mounting it on the base in which the ferrule or the V-groove for mounting the fiber is formed. Further, since no lens is used, there is an advantage that the entire apparatus can be reduced in size.
[0007]
Such a core expansion fiber is made by locally heating a general single mode fiber. The single mode fiber is heated to diffuse a dopant such as Ge doped in the core, thereby widening the diffusion region of the dopant and reducing the relative refractive index difference.
[0008]
If the core diameter is increased in a state where the relative refractive index difference between the core and the clad of the optical fiber is not changed, the single mode condition is broken and the multimode is excited. In the case of the core-enlarging fiber, the diffusion of the dopant due to heat causes simultaneous expansion of the core and reduction of the relative refractive index difference, and r × (D) ^1/2 is automatically kept constant. Here, r is the radius of the core of the optical fiber, D is the relative refractive index difference between the core and the clad, and r × (D) ^1/2 is an amount proportional to the normalized frequency. If this is constant, the single mode condition is Kept.
[0009]
FIG. 7 shows the characteristics of optical coupling using the core expansion fiber. The horizontal axis indicates the distance between the end faces of the core expansion fiber (distance between the opposing surfaces), the vertical axis indicates the coupling loss of light, and w indicates the mode field diameter. The wavelength of light is 1.31 μm generally used in optical communication, and the grooves (between optical fibers) are filled with air (refractive index n = 1). When the core with the mode field diameter of 10 μm is not enlarged, the distance between the optical fibers is 120 μm and the loss is 3 dB or more, whereas when the mode field diameter is 40 μm, the distance between the optical fibers is Even at 900 μm, the loss is 1 dB or less, which clearly shows that the coupling characteristics are improved.
[0010]
However, such a core expansion fiber has the following problems because it is manufactured by heating the optical fiber as described above. In order to expand the core diameter to 40 μm, heating for several hours to several tens of hours is required at a temperature of 1000 ° C. or higher, which is very laborious. Also, the core diameter portion of 10 μm and the portion expanded to 40 μm must be tapered so that the core diameter gradually increases, but it is difficult to control the heating location and temperature distribution. In addition, it is necessary to apply tension so that the optical fiber does not sag during heating, but the optical fiber stretches due to heat and tension, and its outer diameter is slightly reduced, so that the accuracy during alignment is lowered.
[0011]
In addition, an example using a graded index fiber (hereinafter referred to as GI fiber) as a lens is known (for example, see the IEICE General Conference C283).
[0012]
Here, the GI fiber is an optical fiber having an axially symmetric refractive index distribution in which the refractive index gradually decreases from the central axis of the fiber, and is generally used for multimode transmission. Most GI fibers have a refractive index profile of approximately square. Since this refractive index distribution has a lens effect similar to a graded index lens (also called a GRIN lens), a coupling optical system can be formed by using a GI fiber having an appropriate refractive index distribution with an appropriate length. Parameters indicating the characteristics of the GI fiber include a refractive index difference Δ between the cladding and the core center, a core diameter D, and a convergence parameter A.
[0013]
Furthermore, since the light beam in the GI fiber exhibits the behavior of a sine curve as shown in FIG. 9, the length thereof is represented by the pitch (P) corresponding to the cycle of the light beam behavior. The horizontal axis in FIG. 9 represents the pitch, the vertical axis represents the position of the light beam in the GI fiber, and the relative location is shown with 1 being the most diffused light. Note that P = 1 corresponds to one period (2π) of the sine curve. It is P = 0.25 that the point light source becomes parallel light, and it is P = 0.5 that converges again to the point.
[0014]
FIG. 8 shows an example of an optical system using a GI fiber. A GI fiber collimator 12 having a length P = 0.25 (conditions for making the point light source to be collimated light) is joined to the tip of the single mode fiber 1. The GI fiber collimator 12 is aligned on a substrate having a fiber alignment V-groove and an optical element installation groove. Here, if the accuracy of the fiber alignment V-groove is good, there is almost no deviation in the direction perpendicular to the optical axis. That is, since the GI fiber has the same outer diameter as the single mode fiber, if the member for fixing the optical fiber is devised (for example, a ferrule having a high-precision inner diameter or a base having the above-mentioned V-groove), the optical axis However, it is easy to suppress the vertical shift.
[0015]
However, since the GI fiber is a lens, adjustment of the focal direction is necessary and takes time. Moreover, clearance for adjusting the position in the focal direction and mounting the optical element is necessary, and light coupling must be performed once the light is emitted from the optical fiber into the space. When a distance between the GI fibers is required, the adjustment becomes more troublesome. When light is emitted from the GI fiber to the space, there is a problem that reflection occurs at the emission end surface because the refractive index is different between the optical fiber and the space. It was.
[0016]
To solve these problems, a simple structure using an inexpensive GI lens eliminates the need for alignment of all axes, and provides a stable lensless optical fiber body that is less affected by reflection at the end face of the GI fiber. This is an object of the present invention.
[0017]
[Means for Solving the Problems]
In order to achieve the above object, an optical fiber body of the present invention has one end of a graded index fiber connected to one end of a coreless fiber, a single mode fiber connected to the other end of the graded index fiber, and the coreless fiber. One end of another graded index fiber is connected to the other end of the fiber, and one end of another single mode fiber is connected to the other end of the graded index fiber .
[0019]
The length of the coreless fiber is twice the distance from the light exit end face of the graded index fiber to the beam waist.
[0020]
Further, the pitch P that defines the length of the graded index fiber satisfies 0.25 <P <0.5.
[0021]
In addition, the method of manufacturing the optical module of the present invention in which the optical fiber body is disposed on a substrate includes a step of sequentially connecting a graded index fiber and a single mode fiber to both ends of the coreless fiber, and producing an optical fiber body, An optical fiber between the step of installing the optical fiber body on the substrate, the step of forming a groove for dividing the coreless fiber of the optical fiber body into two in the substrate, and the coreless fiber divided in the groove; And a step of interposing an element.
[0023]
Here, when the gap between the end face of the coreless fiber and the optical element is filled with a substance having a refractive index substantially equal to that of the coreless fiber, the interface reflection due to the refractive index difference is eliminated, so that the coupling loss can be minimized.
[0024]
The optical fiber member, when the substrate for fixing the optical element ferrule, or a substrate having a V groove, good because alignment of the optical fiber is facilitated. Further, since the optical isolator is thicker than the wave plate and the filter, the conventional optical system has a large loss. However, according to the present invention, the loss can be reduced and it is suitable for mounting the optical isolator.
[0025]
The collimating condition when the point light source is on the end face of the GI fiber is P = 0.25. However, when the coupling efficiency is actually the highest, the beam waists of the optical fiber bodies coincide with each other. When P = 0.25, the beam waist is positioned just on the exit end face of the GI fiber, and when the optical element is sandwiched between the GI fibers, the beam waists are separated from each other.
[0026]
Therefore, the pitch P needs to be larger than 0.25 in order to make the beam waist away from the end face. Thereby, since the focal length is adjusted in advance by the length of the coreless fiber and the distance between the end faces of the GI fiber is fixed, an optical element can be mounted almost without alignment and a stable optical fiber body with little loss can be obtained.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments according to the present invention will be described below in detail with reference to the drawings. In addition, in each figure, about the same member, the same code | symbol shall be attached | subjected and description shall be abbreviate | omitted.
[0028]
As shown in FIG. 1, the optical fiber body F1 of the present invention has a first single mode fiber 1A for transmission having a mode field diameter (hereinafter referred to as MFD) of about 10 μm, for example, P (pitch)> 0.25. The distance between the beam waist of the light emitted from the first GI fiber 2A and the GI fiber 2A and the light exit end face 15 of the GI fiber 2A is d, the coreless fiber 3, the second GI fiber 2B having a length of 2d, and the transmission Single mode fibers 1B are connected in series to form an optical fiber body F1.
[0029]
That is, one end of the GI fiber 2A is connected to one end of the single mode fiber 1A, and one end of the coreless fiber 3 for adjusting the focal length is connected to the other end of the GI fiber 2A. One end of another GI fiber 2B is connected to the other end of the coreless fiber 3, and one end of another single mode fiber 1B is connected to the other end of the GI fiber 2B. Note that these optical fibers are all made of quartz glass, resin, or the like, and the optical fibers are connected to each other using fusion or a translucent adhesive.
[0030]
Here, the reason why the length of the coreless fiber 3 is twice the distance d from the light emitting end face 15 of the GI fiber 2A to the beam waist is that the optical coupling is maximized.
[0031]
The reason why the pitch P that defines the length of the GI fibers 2A and 2B is 0.25 <P <0.5 is that there is a beam waist on the outside (coreless fiber side) of the GI fiber. is there. When P <0.25, the beam waist is in the GI fiber, and the emitted light becomes divergent light.
[0032]
Next, an optical module provided with such an optical fiber body F1 will be described. As shown in FIG. 2A, the first GI fiber 2A is connected to the tip of the first single mode fiber 1A by fusion or a light-transmitting adhesive. Next, as shown in FIG. 2B, the coreless fiber 3 is similarly connected to the GI fiber 2A. As shown in FIG. 2C, the second GI fiber 2B is connected to the second transmission fiber 2B. The single mode fiber 1B is connected in the same manner. Next, as shown in FIG. 2 (d), the optical fiber body F1 is mounted on the substrate 5 on which the V-groove 13 for fixing the fiber is accurately formed by anisotropic etching or the like, and fixed with an adhesive. To do. Then, as shown in FIG. 2 (e), an optical element mounting groove 14 for dividing the coreless fiber 3 is formed by dicing in the middle portion of the coreless fiber 3, and as shown in FIGS. 2 (f) and 2 (g), the wavelength filter An optical element 4 such as an optical isolator is disposed, and a gap 16 between the coreless fiber 3 and the optical element 4 is filled and fixed with a translucent adhesive 7 having a refractive index substantially equal to that of the coreless fiber.
[0033]
In this way, the optical fiber body F1 is arranged on the base body 5, the groove 14 for dividing the coreless fiber 3 into two is formed in the base body 5, and the optical element 4 for optically connecting the coreless fibers 3 divided in the groove 14 A very excellent optical module M1 with a small loss is completed.
[0035]
【Example】
Hereinafter, more specific examples of the present invention will be described.
[0036]
[Example 1]
First, as shown in FIG. 2 (a), Δ = 0.85%, the core diameter is 105 μm, and the convergence parameter A = 3.37 × 10 ^{−6 at} the tip of the silica-based single mode fiber 1A having an MFD of about 10 μm. A GI fiber 2A of μm ⁻² and P = 0.258 (653 μm) was connected by fusion bonding by electric discharge.
[0037]
If the surrounding medium is n = 1.46 (equivalent to the refractive index of the optical fiber), the distance from the end face 15 of the GI fiber 2a to the beam waist of the emitted light formed by this GI fiber is 550 μm.
[0038]
As shown in FIG. 2B, the coreless fiber 3 having a refractive index of n = 1.46 was connected to the GI fiber 2A by fusion bonding by electric discharge, and cut to a length of 1100 μm. Next, as shown in FIG. 2 (c), the same GI fiber 2B and single mode fiber 1B as the GI fiber 2A were fusion spliced in this order to produce an optical fiber body F1.
[0039]
Next, as shown in FIG. 2D, anisotropic etching with a KOH aqueous solution is performed on a substrate made of single crystal silicon having a (100) plane as a principal surface with a Miller index, and a width of 140 μm and a Miller index {111 } A fiber mounting V-groove 13 having a sloped surface is formed, and an optical fiber body F1 is placed on the base 5 (length 5 mm, width 3 mm, thickness 1 mm), and a thermosetting adhesive that is an epoxy resin is used. This was fixed.
[0040]
Next, as shown in FIG. 2 (e), an optical element mounting groove 14 (800 μm in width) was formed by cutting with a dicer to divide the coreless fiber 3. Then, as shown in FIG. 2 (f), an optical element 4 (optical isolator) having a thickness of 700 μm is installed in the optical element mounting groove 14 and is an ultraviolet resin which is an epoxy resin adjusted to a refractive index n = 1.46. The curable refractive index matching adhesive was filled and fixed in the gap between the optical element 4 and the coreless fiber 3 without any gap. The insertion loss at this time was 0.51 dB together with the optical element.
[0041]
In this embodiment, the thermosetting adhesive, which is an epoxy resin, is used for fixing the optical fiber and the substrate. However, a more reliable low melting point glass or solder may be used.
[0042]
[Example 2]
FIG. 3 shows an embodiment in which the optical fiber body F1 of the present invention is used and a ferrule is used as a base. An optical fiber F1 produced in the same manner as in Example 1 was inserted and fixed in a zirconia ferrule 6 having a diameter of 1.25 mm and a length of 10 mm. A thermosetting epoxy adhesive was used for fixing the optical fiber.
[0043]
Further, an optical element mounting groove 14 having a width of 800 μm was formed in the ferrule 6 at a position where the coreless fiber 3 was divided by a dicer. Then, an optical element 4 (optical isolator) having a thickness of 700 μm was inserted into the optical element mounting groove 14 and filled and fixed with an ultraviolet curable adhesive 7 having a refractive index n = 1.46. The insertion loss at this time was 0.44 dB including the loss of the optical element.
[0044]
Further, the inner diameter accuracy of the ferrule 6 is guaranteed on the order of submicrons, and since it is held from the entire circumferential direction of the fiber, the coaxiality and the optical straightness of the fiber are superior to the V-grooved substrate. Further, since the coreless fiber 3 that has been integrated is divided, no axial deviation occurs in principle.
[0045]
[Example 3]
In the optical system assembled in the ferrule of Example 2, two GI fiber collimators are used and the optical element is sandwiched and fixed.
[0046]
As shown in FIG. 4, at the tip of the single mode fiber 1A, Δ = 0.85%, core diameter 105 μm, convergence parameter A = 3.37 × 10 ⁻⁶ μm ⁻² , P = 0.258 (653 μm). The GI fiber 2A was connected by fusion bonding by electric discharge.
[0047]
If the surrounding medium is n = 1.46 (equivalent to the refractive index of the optical fiber), the distance from the end face 15 of the GI fiber 2A to the beam waist of the emitted light formed by this GI fiber is 550 μm.
[0048]
Since an optical element having a thickness of 700 μm is mounted, a coreless fiber 3A having a length of 200 μm (550−700 / 2) and a refractive index of n = 1.46 is connected to the GI fiber 2A by fusion bonding by electric discharge. Thus, a GI fiber collimator 12 was produced.
[0049]
A micro optical isolator 4 processed into a cylindrical shape having a diameter of 120 μm and a thickness of 700 μm is inserted into a zirconia ferrule 6 having a diameter of φ1.25 mm and a length of 10 mm, and a refractive index n = from both ends of the ferrule 6 to the GI fiber collimator 12. 1.46 thermosetting epoxy adhesive was applied and cured to fix the optical isolator 4.
[0050]
Although the end face of the optical fiber is cleaved at the time of cutting, the loss due to surface scattering can be reduced because the smoothness is higher than the cut surface or polished surface by the dicer. Further, since the GI fiber collimator 12 is abutted against the optical element (optical isolator 4), the gap can be minimized.
[0051]
【The invention's effect】
As described in detail above, according to the optical fiber body of the present invention, the following remarkable effects can be obtained.
[0052]
-Since no lens is used, it can be manufactured at a low cost with a simple configuration.
[0053]
-Since the optical system can be formed only by adjusting the length of the GI fiber, the adjustment axis is small and the optical element can be easily arranged (the alignment method is not required for the dividing method).
[0054]
-It is excellent in stability because it is fixed with a coreless fiber whose focal length has been adjusted in advance.
[0055]
・ Because optical adjustment can be performed first by adjusting the length of the optical fiber, and the optical element can be mounted without alignment, the optical fiber can be fixed at a high temperature such as solder or low-melting glass even if the optical element itself is not heat resistant. It can be fixed by the method. Normally, since the lens and the optical fiber are adjusted after the optical element is installed, a high temperature process cannot be used for fixing the optical fiber.
[0056]
Coreless fiber has a higher refractive index than air (n = 1), so there is less beam spread and therefore greater coupling efficiency and tolerance.
[0057]
-By filling a refractive index matching agent between the optical element and the optical fiber, light is not reflected at the end face of the optical fiber. Further, since the gap is filled, no condensation or contamination occurs on the end face of the optical element or fiber.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view schematically illustrating an optical fiber body according to the present invention.
FIGS. 2A to 2G are views for schematically explaining a manufacturing process of an optical fiber body according to the present invention, FIG. 2A to FIG. 2F are perspective views, and FIG. is there.
FIG. 3 is a cross-sectional view schematically showing an embodiment of an optical fiber body according to the present invention.
FIG. 4 is a cross-sectional view schematically showing an embodiment of an optical fiber body according to the present invention.
FIG. 5 is a cross-sectional view schematically illustrating a conventional optical system.
FIG. 6 is a cross-sectional view schematically illustrating a conventional optical system.
FIG. 7 is a graph showing the relationship between the facing distance of the core expansion fiber and the coupling loss.
FIG. 8 is a cross-sectional view schematically illustrating a conventional optical system.
FIG. 9 is a schematic diagram for explaining the behavior of light rays in a GI fiber.
[Explanation of symbols]
1A, 1B: Single mode fiber 2A, 2B: GI fiber 3: Coreless fiber 4: Optical element 5: Substrate 6: Ferrule 7: Refractive index matching adhesive 8: Lens 9: Holder 10: Package 11: Core expansion fiber 12: GI fiber collimator 13: fiber fixing V groove 14: optical element mounting groove 15: end face 16: gap F1: optical fiber body M1: optical module

Claims (2)

  Connect one end of the graded index fiber to one end of the coreless fiber, connect the single mode fiber to the other end of the graded index fiber, and connect one end of the other graded index fiber to the other end of the coreless fiber Then, one end of another single mode fiber is connected to the other end of the graded index fiber, and the pitch P defining the length of the graded index fiber satisfies 0.25 <P <0.5. The length of the coreless fiber is twice the distance from the light exit end face of the graded index fiber to the beam waist. A method of manufacturing an optical module in which the optical fiber body according to claim 1 is disposed on a base, wherein a graded index fiber and a single mode fiber are respectively connected to both ends of a coreless fiber, and an optical fiber body is manufactured. , A step of installing the optical fiber body on a base, a step of forming a groove for dividing the coreless fiber of the optical fiber body into two in the base, and an optical element between the coreless fibers divided in the groove And a step of interposing an element.

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